Vehicles having automatic transmissions are typically equipped with a driver manipulated transmission range selector positionable to one of a number of sectors for indicating a desired operating range of the transmission. The forward operating range is defined by a Drive sector and one or more manual pulldown sectors corresponding to the various forward speed ranges provided by the transmission.
To operate the vehicle in a forward range, the selector is ordinarily moved to the Drive sector, and the speed ratio selection is carried out automatically in accordance with an empirically determined shift pattern based on engine load (throttle position) and vehicle speed. For any given engine load, for example, the shift pattern may dictate a first vehicle speed above which an upshift should be initiated, and a second vehicle speed below which a downshift should be initiated.
However, the predetermined shift pattern is not especially suited to operation in hilly terrain. During such operation, the driver is encouraged to move the range selector to a manual pulldown position. In addition to downshifting the transmission to the indicated speed ratio, the manual pulldown configures the transmission to enable engine braking and inhibits upshifting to a higher speed ratio.